Bottoms up, bubbles down.

• Position: Physics - Bubbles in beer

An experiment finally has proven what beer lovers long have suspected: When the alcoholic beverage is poured into a glass, the bubbles sometimes go down instead of up, confirm chemists from Stanford (Calif.) University and the University of Edinburgh, Scotland.

"Bubbles are lighter than beer, so they're supposed to rise upward," explains Richard N. Zare, the Marguerite Black Wilbur Professor in Natural Sciences at Stanford, "But countless drinkers have claimed that the bubbles actually go down the side of the glass."

This frothy question reached a head in 1999 after Australian researchers announced that they had created a computer model showing that it was theoretically possible for beer bubbles to flow downward. The Australians based their simulation on the motion of bubbles in a glass of Guinness draught--a popular Irish brew that contains both nitrogen and carbon dioxide gas.

However, Zare and former Stanford postdoctoral fellow Andrew J. Alexander--now a professor at Edinburgh--were skeptical of the virtual Guinness model and decided to put it to the test by analyzing several liters. "Andy got a hold of a camera that takes 750 frames a second and recorded some rather gorgeous video clips of what was happening," notes Zare. A careful analysis of the video confirmed the Australian team's findings: Beer bubbles can and do sink to the bottom of a glass--but why?

"The answer turns out to be really very simple," Zare relates. "It's based on the idea of what goes up has to come down. In this case, the bubbles go...